Journal of Agricultural Research 
Vol. II, No. a 
156 
consequently that of the hops. It is usually acknowledged that hops of 
foreign origin possess a more agreeable odor than American hops. The 
difference in odor always seems to be perceptible although the odor is 
closely related to that of American hops. That the difference in odor is 
due to a difference in the volatile oil present can scarcely be questioned; 
in fact, it has been shown that such is the case. This is not, however, the 
only instance among aromatic plants where geographical source, with 
the different conditions of climate and soil, shows its effect in the differ¬ 
ences in volatile oils distilled from the plants, as, for example, lavender 
and peppermint. Not only do hops of foreign origin produce oils notice¬ 
ably dissimilar in some of their properties from the American oils, but 
hops grown even in separated sections of the United States have a differ¬ 
ent odor and yield oils with more or less constant differences from year 
to year. 
CONCLUSIONS 
The volatile oil of hops has been shown to consist chiefly of the terpene 
myrcene, the heptoic, octoic, and nonoic acid esters of the alcohol myr- 
cenol, and the sesquiterpene humulene, with traces of free acids, formal¬ 
dehyde, and probably some free alcohols. The constituents of chief 
importance as regards odor are the above-named esters, which constitute 
a large portion of the oil. 
The several oils examined have been found to contain varying propor¬ 
tions of the esters as well as the terpene myrcene and the sesquiterpene 
humulene. Although no great importance can be attached to the two 
latter constituents, the variable content of esters is most significant, 
since the quality of the odor is probably most greatly influenced by them. 
Important differences in the oils are apparent not only during any 
particular season but for several seasons. These constant differences 
are shown most forcibly in the curves of the physical and chemical prop¬ 
erties of the oils. The curves of fractional distillation, which may be 
said to represent a partial quantitative separation of the chief constitu¬ 
ents, bring out strongly the relationship which exists between the hops 
from any source during one season or several seasons. The optical rota¬ 
tion curves also show this relationship. In general the physical proper¬ 
ties of the oils—the fractionation, specific gravity, and optical rotation— 
show strong similarities which may exist among related oils or strong 
dissimilarities among unrelated oils. The esters being regarded as the 
constituents of most importance in affecting the odor of the hops from 
which the oils were distilled are compared by means of the ester numbers. 
The curves of the ester content of the various oils and fractions of the 
oils show at a glance the remarkable differences in the oils from the 
several geographical sources. The courses followed by the imported 
oils are most conspicuous because of their constantly lower ester content. 
More remarkable is the fact that the ester content of the imported oils 
